The giant trevally (Caranx ignobilis), known in Hawaii as ‘ulua aukea’, is a large (up to 170 cm Total Length, 80 kg) top predator found on coral reefs across the Indo-Pacific region where it is highly sought after as a food and game fish. These highly targeted fishes are estimated to contribute tens of millions of dollars annually to the Hawaii economy, but their numbers have become noticeably depleted in recent decades. We need to understand their movement patterns in order to determine appropriate management strategies to help restore their populations.

We are using an array of underwater listening receivers to study the long-term movements of giant trevally and answer key management questions, such as how far they range and what are their typical patterns of movement. This technology has already provided unprecedented insight into the scale and patterns of giant trevally movements in the remote North West Hawaiian Islands (Papahanaumokuakea Marine National Monument). Giant trevally at remote NWHI atolls are site-attached to home ranges where they swim back and forth daily between separate day and night habitats. During the full moons in summer, giant trevally migrate up to 30km to reach traditional spawning grounds where they form large aggregations. We are now using this system to determine whether giant trevally show similar patterns of behavior in the Main Hawaiian Islands (MHI).

Research Questions

We are asking four specific questions about giant trevally long-term movements in the Main Hawaiian Islands:

Are MHI giant trevally site-attached to permanent home ranges?

Do MHI giant trevally exhibit daily migrations between separate day and night habitats?

The giant trevally movement data obtained in this study will be used to shape management strategies for this important food and game fish.

Methods

To date we have captured 18 giant trevally off the Kona (west) coast of Hawaii Island and surgically implanted them with small acoustic transmitters. We are monitoring their movements with the West Hawaii listening array consisting of 37 underwater receivers deployed along 109km of coastline. These receivers identify and record the presence of any acoustic transmitters within range (up to 250 m). The receivers have a battery life of approximately 15 months and are being downloaded at 3-6 month intervals.

Ulua tracked in the MHI exhibited strong diel, lunar and seasonal patterns of movement, similar to those previously documented in the North Western Hawaiian Islands (NWHI). Tracked fish were highly site-attached to relatively small (<5km in length) home ranges during the winter months and migrated up to 38 km south along the west Hawaii coast during summer. These summer movement patterns are consistent with ulua spawning migrations in the NWHI. Active tracking revealed that ulua ranged primarily between depths of 50 and 250 meters. This is below the depth of conventional SCUBA diving and spearfishing, hence ulua may enjoy a partial ‘refuge-in-depth’ from fishing mortality in the MHI. Additional research is required to determine the location of ulua spawning grounds on the west Hawaii coast, and to determine whether ulua from all around Hawaii island migrate to the same spawning area.

Figure 2. Horizontal (left) and vertical (right) movements of a giant trevally captured outside Kealakekua Bay. Daytime and nighttime movements are indicated in yellow and black respectively. This tracks was obtained after the fish had been at liberty for 74 days after initial capture and tagging.